Heterogeneous computer system, heterogeneous input output system and data back-up method for the systems

ABSTRACT

In a heterogeneous computer system, a heterogeneous input/output system and a data back-up method for the systems, an I/O subsystem A for an open system and an I/O subsystem B for a mainframe are connected by a communication unit. In order to back up data from at least one disk connected to the I/O subsystem B in a magnetic tape library system and in order to permit the mainframe to access the data in the I/O subsystem B, the I/O subsystem A includes a table for assigning a vacant memory address in a local subsystem to the memory of the I/O subsystem for an open system. A request of variable-length record format received from the mainframe is converted into a fixed-length record format for the subsystem B. The disk designated according to the table is accessed, and the data thus obtained is sent to the mainframe and backed up in the back-up system.

This application is a continuation of application Ser. No. 09/052,985,filed Apr. 1, 1998 now U.S. Pat. No. 6,098,129.

BACKGROUND OF THE INVENTION

The present invention relates to a heterogeneous computer systemcomprising a host computer and a plurality of I/O subsystems, and morein particular to a method for making it possible to back up the datastored in a memory between a host computer and an I/O subsystem whichcannot be directly connected due to the difference in access interface,and a heterogeneous computer system including a plurality of I/Osubsystems having different access interfaces connected to the systemand the host computer.

In mainframes, a large scale of memory hierarchy (storage hierarchy)including a combination of a plurality of external memories havingdifferent processing speeds and different storage capacities isaccompanied by a satisfactory data management function and an overallstorage management function intended to support an optimum dataarrangement and an efficient operation. The IBM's DFSMS (Data FacilityStorage Management Subsystem) is an example, which is described indetail in “IBM SYSTEMS JOURNAL, Vol. 28, No. 1, 1989, pp.77-103.

The disk data of the I/O subsystem of the mainframe computer systemhaving the above-mentioned management function can be backed up in amedium such as a magnetic tape or a magnetic tape library capable ofstoring a large quantity of data with a low cost per bit.

An open system such as a personal computer or a work station, unlike themainframe, is not equipped with a magnetic tape or a magnetic tapelibrary capable of storing a large quantity of data.

Generally, in an open system such as a personal computer or a workstation, a disk is accessed in accordance with a fixed-length recordformat, while the mainframe accesses a disk in accordance with avariable-length record format called the count key data format.

As a result, the disk subsystem for the mainframe computer is oftenconfigured independently of the disk subsystem for the open system.

On the other hand, a technique for transmitting and receiving databetween I/O subsystems is disclosed in U.S. Pat. No. 5,155,845.

In a disk subsystem for an open system and a disk subsystem for amainframe computer which use different host computers, the back-up andother functions are independently operated and managed.

In view of the fact that the open system lacks medium such as a magnetictape or a magnetic tape library capable of storing a larger quantity ofdata, as described above, it is effective to back up the data in the I/Osubsystem of the mainframe.

An ordinary disk system for the open system, however, cannot beconnected directly to the mainframe due to the difference in theinterface thereof.

U.S. Pat. No. 5,155,845 fails to disclose how to process the read/writeoperation for a storage system not directly connected to a hostcomputer.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and a systemfor backing up data stored in a memory between a host computer and anI/O subsystem that cannot be connected directly to each other due to thedifference in access interface.

Specifically, an object of the invention is to provide a method and asystem for backing up data stored in an I/O subsystem of an open systemfrom a mainframe not directly connected to the I/O subsystem.

Another object of the invention is to provide a method and a computersystem in which a mainframe is capable of accessing a memory of an I/Osubsystem of an open system not directly connected to the mainframe.

Still another object of the invention is to provide a system and amethod of access in which two or more I/O subsystems having differentinterfaces can be connected to a mainframe.

In order to achieve the above-mentioned objects, according to one aspectof the present invention, there is provided a heterogeneous computersystem comprising a first host computer, a first I/O subsystem directlyconnected to the first host computer by an interface of variable-lengthrecord format and including at least one external memory, a second hostcomputer, a second I/O subsystem directly connected to the second hostcomputer by an interface of fixed-length record format and including atleast one external memory, and a communication unit for connecting thefirst I/O subsystem to the second I/O subsystem;

wherein the first I/O subsystem includes a table for storing a deviceaddress of an external memory, data indicating one of the externalmemory of the first I/O subsystem and the external memory of the secondI/O subsystem to which the device address is assigned, and a deviceaddress of the external memory in the second I/O subsystem when thedevice address is assigned to the external memory of the second I/Osubsystem; and

wherein upon receipt of a read/write request conforming to the interfaceof variable-length record format from the first host computer andincluding an address of an external memory to be read from or writteninto, and upon decision, with reference to the table, that the externalmemory address included in the read/write request is assigned to theexternal memory included in the second I/O subsystem, the first I/Osubsystem converts the read/write request into a second read/writerequest conforming to the interface of fixed-length record format andsends the second read/write request to the second I/O subsystem.

According to another aspect of the invention, there is provided aheterogeneous computer system comprising a first host computer, a firstI/O subsystem directly connected to the first host computer by aninterface of variable-length record format and including at least oneexternal memory, a back-up system connected to the first host computer,a second host computer, a second I/O subsystem directly connected to thesecond host computer by an interface of fixed-length record format andincluding at least one external memory, and a communication unit forconnecting the first I/O subsystem to the second I/O subsystems;

wherein the first host computer includes a means for issuing to thefirst I/O subsystem a read request conforming to the interface ofvariable-length record format and containing the address of an externalmemory from which data is to be read, and backing up the data receivedfrom the first I/O subsystem into the back-up system;

wherein the first I/O subsystem includes a table for storing the deviceaddress of an external memory, data indicating that one of the externalmemory of the first and the second I/O subsystems to which the deviceaddress is assigned, and the device address of the external memory inthe second I/O subsystem when the first device address is assigned tothe external memory of the second I/O subsystem; and

wherein upon receipt from the first host computer of a read requestconforming to the interface of variable-length record format includingan external memory address to be read, and upon decision, with referenceto the above-mentioned table, that the device address of the memoryaddress included in the read request is assigned to the external memoryincluded in the second I/O subsystem, the first I/O subsystem convertsthe read request into a second read request conforming to fixed-lengthinterface and sends the second read request to the second I/O subsystemwhile at the same time sending to the first host computer the datareceived from the second I/O subsystem.

According to still another aspect of the invention, there is provided aheterogeneous computer system comprising a first host computer, a firstI/O subsystem directly connected to the first host computer by aninterface of variable-length record format and including at least oneexternal memory, a back-up system connected to the first host computer,a second host computer, a second I/O subsystem directly connected to thesecond host computer by an interface of fixed-length record format andincluding at least one external memory, and a communication unit forconnecting the first I/O subsystem to the second I/O subsystem;

wherein the first host computer includes a means for issuing to thefirst I/O subsystem a write request conforming to the interface ofvariable-length record format including the address of an externalmemory into which data is to be written, and sending the data read fromthe back-up system to the first I/O subsystem;

wherein the first I/O subsystem includes a table containing a deviceaddress of the external memory, data indicating one of the externalmemories of the first and the second I/O subsystems to which the deviceaddress is assigned, and a device address of the external memory in thesecond I/O subsystem when the first device address is assigned to theexternal memory of the second I/O subsystem; and

wherein upon receipt from the first host computer of a write requestconforming to the interface of variable-length record format includingthe device address of an external memory to be written into, and upondecision, with reference to the table, that the address of the externalmemory included in the write request is assigned to the external memoryincluded in the second I/O subsystem, the first I/O subsystem convertsthe write request into a second write request conforming to theinterface of fixed-length record format, sends the second write requestto the second I/O subsystem while at the same time sending the datareceived from the first host computer to the second I/O subsystem.

According to yet another aspect of the invention, there is provided aheterogeneous I/O system for use with a host computer connected thereto,comprising a first I/O subsystem including at least one external memory,and a second I/O subsystem connected to the first I/O subsystem andincluding at least one external memory;

wherein the first I/O subsystem includes a table for storing a deviceaddress of an external memory, data indicating one of the externalmemories of the first and the second I/O subsystems to which the deviceaddress is assigned, and a device address of the external memory in thesecond I/O subsystem when the first device address is assigned to theexternal memory of the second I/O subsystem;

wherein upon receipt of a read/write request designating the deviceaddress of an external memory to be read from or written into by thehost computer, and upon decision, with reference to the table, that theexternal memory address in the designated address is assigned to theexternal memory included in the second I/O subsystem, the first I/Osubsystem sends the read/write request to the second I/O subsystem.

According to a further aspect of the invention, there is provided aheterogeneous I/O system for use with a host computer connected thereto,comprising a first I/O subsystem having an interface of variable-lengthrecord format and including at least one external memory, a second I/Osubsystem having an interface of fixed-length record format andincluding at least one external memory, and a communication unit forconnecting the first I/O subsystem to the second I/O subsystem;

wherein the first I/O subsystem includes a table for storing a deviceaddress of an external memory, data indicating one of the externalmemories of the first and the second I/O subsystems to which the deviceaddress is assigned, and a device address of the external memory in thesecond I/O subsystem when the first device address is assigned to theexternal memory of the second I/O subsystem; and

wherein upon receipt from the host computer of a read/write requestconforming to the interface of variable-length record format includingthe address of an external memory to be read from or written into, andupon decision, with reference to the table, that the external memoryaddress included in the read/write request is assigned to the externalmemory included in the second I/O subsystem, the first I/O subsystemconverts the read/write request into a second read/write requestconforming to the interface of fixed-length record format and sends itto the second I/O subsystem.

According to an embodiment of the invention, there is provided aheterogeneous computer system, wherein an I/O subsystem for an opensystem is connected to an I/O subsystem for a mainframe by acommunication unit, wherein, in order to access data in the I/Osubsystem for an open system from the mainframe for enabling the data inthe disk connected to the I/O subsystem for the open system to be backedup in a magnetic tape library system; a table is prepared for assigninga vacant address of the memory in the local subsystem to the memory ofthe I/O subsystem for the open system; wherein a request ofvariable-length record format received from the mainframe is convertedinto a request of fixed-length record format for the open system;wherein the disk designated according to the table is accessed, andwherein the data thus obtained is sent to the mainframe and backed up inthe back-up system.

This configuration can back up the data of an I/O subsystem for an opensystem in a back-up system under the management of a mainframe notdirectly connected to the particular I/O subsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a heterogeneouscomputer system according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a heterogeneouscomputer system according to another embodiment of the invention.

FIG. 3 is a block diagram showing a configuration of a disk controllerof the heterogeneous computer system shown in FIGS. 1 and 2.

FIG. 4 is a diagram showing a configuration of a localcontroller-connected disk data (table) for the systems shown in FIGS. 1and 2.

FIG. 5 is a diagram showing a configuration of a remotecontroller-connected disk data (table) for the systems shown in FIGS. 1and 2.

FIG. 6 is a diagram showing the interconnection of disk devices asviewed from the mainframe.

FIG. 7 is a diagram showing an example of the processing flow of a diskcontroller A in the case where the data in an I/O subsystem for an opensystem is backed up in an MT library system of the mainframe.

FIG. 8 is a diagram showing an example of the processing flow of a diskcontroller A in the case where data are restored in an I/O subsystem foran open system from an MT library system of the mainframe.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be described below with reference tothe accompanying drawings.

FIG. 1 is a diagram showing a configuration of a computer systemaccording to an embodiment of the invention.

A processing system A 100 includes a mainframe 101, a channel interfaceA 102, a channel interface B 103, a magnetic tape (MT) controller 106, amagnetic tape library controller 130, a magnetic tape library 107, adisk controller A 104, a disk drive group A 105 and a service processorA 109. A back-up processing device 162 and a restore processing device164 are mounted on the mainframe 101.

The mainframe 101 accesses the disk controller A 104 through the channelinterface B 103 conforming with a variable-length record format calledthe count-key-data format.

The count-key-data format is a record format in which a recordconstituting a unit of read/write operation is configured of threefields including a count field, a key field and a data field.

A record ID is stored in the count field, a key data for accessing therecord is stored in the key field, and the data used by an applicationprogram is stored in the data field.

In the description that follows, the magnetic tape (MT) controller 106,the magnetic tape library controller 130 and the magnetic tape library107 are collectively referred to as an MT library system 116. The diskcontroller A 104 and the disk drive group A 105 constitute an I/Osubsystem 10 connected to the mainframe 101. In similar fashion, thedisk controller B 113 and the disk drive group B 114 constitute an I/Osubsystem 20 connected to a host 111 for an open system.

An optical disk or the like, as well as a magnetic disk, constitutes arank of storage hierarchy connected through the channel interface. Thefollowing description refers to the case in which the MT library system116 is connected.

The disk controller A 104 contains local controller-connected disk data314 and remote controller-connected disk data 315.

The local controller-connected disk data 314 and the remotecontroller-connected disk data 315 are data provided for making itpossible for the mainframe to access a disk device of the I/O subsystemnot directly connected thereto. Specifically, the data 314 and 415 are atable for assigning a vacant address of the memory in the local I/Osubsystem 10 for the processing system A to the memory of the I/Osubsystem for the open system so that the data in the I/O subsystem 20for the processing system B can be accessed from the mainframe 101. Thedata 314 and 315 will be described in detail later.

The processing system B 110 includes a host 111 for the open system, aSCSI (small computer system interface) 112, the disk controller B 113,the disk drive group B 114 and a service processor B 115.

The host 111 for the open system accesses the disk controller B 113through the SCSI 112 having a fixed-length record which is a unit ofread/write operation.

The disk controller A 104 and the disk controller B 113 are connected bya communication line 108. The communication line 108 can be, forexample, a SCSI cable B 117.

In the description that follows, the count-key-data format will becalled the CKD format, and the fixed-length block format will be calledan FBA (fixed block architecture) format.

Also, the record of the CKD format will be referred to as the CKDrecord, and the record of the FBA format will be referred to as the FBArecord.

FIG. 2 is a diagram showing another example of a computer systemaccording to the invention, in which a single I/O subsystem for themainframe is connected to two or more I/O subsystems for an open system.

In a processing system X 120, the interfaces of an open system host X121 and a disk controller X 123 are connected to each other by a fiberchannel interface 122. The fiber channel interface 122 is an opticalfiber cable which can increase the length of connection between a hostand a control device.

In many case, however, a fiber channel interface based on SCSI isemployed between a host and a control device.

Also, an interface such as a fiber channel interface X 126 can be usedto connect a disk controller X 123 and the disk controller B 113.

The data back-up system in the configuration of FIG. 2 is an expansionof the data back-up system in the configuration of FIG. 1.

The fundamental operation of each system is such that the mainframe 101and the hosts 111 and 121 for the open system access the magnetic tapelibrary 107 constituting an external memory or the disk drive group A105, the disk drive group B 114 and the disk drive group X 124 througheach interface. The process in the mainframe 101 establishes a route tothe data stored externally through each interface under the control ofan arbitrary operating system such as Hitachi's VOS3 (virtual-storageoperating system 3) for supporting the channel interface, while theprocess in the host for the open system establishes a route to theexternally-stored data through each interface under the control of anarbitrary operating system such as UNIX (a registered trade mark ownedby X/Open in U.S.A. and other countries) for supporting the SCSI.

FIG. 3 is a diagram showing a configuration of the disk controller A104.

The disk controller A 104 includes a MPU 302 for executing a controlsystem process 307 of the disk controller, a memory 301, a host datatransfer device 303, a disk/cache device 304, an inter-I/O subsystemdata transfer device 305, a data transfer device 306 and a control bus308 for connecting these devices.

The control system process 307 operates in a multitask or multiprocessorenvironment.

The memory 301 includes various microprograms 312 and various data 313.

Especially, the disk controller A 104 has stored therein the localcontroller-connected data 314 and the remote controller-connected diskdata 315, as described above with reference to FIG. 1.

The disk controller B 113 and the disk controller X 123 have aconfiguration similar to the disk controller A 104 and will not bedescribed in detail.

The disk controller B 113 and the disk controller X 123, however, arenot required to contain the local controller-connected disk data 314 andthe remote controller-connected disk data 315.

The local controller-connected disk data 314 is the data indicating theconnections of the controllers and the like, and stored in the memory301 of the disk controller A 104. The local controller-connected diskdata 314 exists as the data corresponding to each disk device.

The local controller-connected disk data 314 is shown in FIG. 4.

The device address 400 is an identifier (ID) for discriminating a diskdevice to be read from or written into by a host computer such as themainframe 101, and is the data also contained in the read/write requestissued by the host computer such as the mainframe 101.

Local controller connection data 401 is the data indicating whether ornot the disk drive corresponding to the controller-connected disk data314 is actually connected to a controller.

A remote controller connection pointer 402 indicates whether or not thecontroller-connected disk data 314 is assigned to a disk drive connectedto a remote controller.

In the case where the such data is assigned to a disk drive connected toa remote controller, the pointer indicates a corresponding remotecontroller-connected disk data 315. Otherwise, the pointer assumes anull value.

In the case where the remote controller connection pointer 402 is valid(i.e. in the case where the particular device address 400 is assigned toa disk device connected to a remote controller), it represents the statein which the local controller connection data 401 is not assigned.

In the case where the remote controller connection pointer 402 isinvalid (i.e. in the case where the device address 400 is not assignedto a disk drive connected to a remote controller), on the other hand,the local controller connection data 401 may indicate the state ofno-assignment.

In other words, the device address 400 may be assigned to neither a diskdevice connected to a local controller nor a disk device connected to aremote controller.

An attribute 403 is the data unique to a device including the interface,the function, the data format and the block length of the disk drive.

The local controller-connected disk data 315 shown in FIG. 5 is the datacorresponding to a disk drive not directly connected to the diskcontroller A 104.

It follows therefore that the remote controller-connected disk data 315,on the other hand, is pointed to by any one of the localcontroller-connected disk data 314.

A connection controller address 500 represents the address of acontroller connected with a disk device corresponding to the remotecontroller-connected disk data 315. According to this embodiment, theaddress of the disk controller B 113 is stored as the connectioncontroller address 500.

A disk address 501 represents the address assigned in the controlleractually connected to a corresponding disk drive.

The local controller-connected disk data 314 and the remotecontroller-connected disk data 315 are set from the service processor109.

According to this embodiment, the mainframe 101 recognizes that the diskdrive group B 114 (disks C and D) is also connected to the diskcontroller A 104 through the disk controller B 113, as shown in FIG. 6,taking advantage of the local controller-connected disk data 314 and theremote controller-connected disk data 315 shown in FIGS. 4 and 5.

This is because of the fact that the vacant address of disk driveavailable in the disk controller A 104 is assigned by the diskcontroller A 104 to a disk drive of the I/O subsystem for an opensystem.

Now, the back-up processing will be described with reference to FIGS. 1,7 and 8.

Specifically, in FIG. 1 the back-up process 162 on the mainframe 101causes the data in the disk device group B 114 of the open system of theprocessing system B to be backed up in the MT library system 116 throughthe disk controller A 104 and the mainframe 101 of the processing systemA.

Conversely, the data backed up in the MT library system 116 is restoredin the disk drive group B 114 of the open system of the processingsystem B through the mainframe 101 and the disk controller A 104 of theprocessing system A.

The back-up operation and the restoration described above are executedin response to a command from the mainframe 101.

First, an explanation will be given of the case in which the data in thedisk drive group B 114 of the open system for the processing system B isbacked up in the MT library system 116 through the disk controller A 104and the mainframe 101 of the processing system A.

As already described above, the mainframe 101 has recognized that thedisk drive group B 114 (disks C and D) are also connected to the diskdrive A 104.

Therefore, the operation of the mainframe 101, which is simply to issuea read request to the disk controller A 104 and back up the receiveddata in the MT library system 116, will not be described specifically.

In the case of backing up data into the MT library system 116, themainframe 101 issues a read request to the disk controller A 104. Thedisk controller A 104 executes the process in accordance with theflowchart of FIG. 7 in response to a read request from the mainframe101.

First, step 700 finds out a corresponding local controller-connecteddisk data 314 from the address of the disk drive designated in the readrequest.

Step 701 checks whether the designated disk drive is connected to thedisk controller A 104 or not.

In the case where the disk drive is connected to the disk controller A104, step 702 reads the corresponding data from the particular diskdrive.

In the case where the disk drive is not connected to the disk controllerA104, in contrast, step 703 checks whether the designated disk drive isconnected to a remote disk controller (disk controller B 113). In otherwords, it checks whether the remote controller connection pointer 402assumes a null value.

In the case where the check result shows that the remote controllerconnection pointer 402 assumes a null value indicating that thedesignated disk drive is not connected to the remote disk controller, anerror is reported in step 704.

The operation specifically related to the invention is represented bystep 705 and subsequent steps executed in the case where a designateddisk drive is connected to a remote disk controller (disk controller B113).

First, in the case where the check result shows that the remotecontroller connection pointer 402 does not assume the null valueindicating that the designated disk drive is connected to a remote diskcontroller, step 705 finds out the remote controller-connected disk data315 corresponding to the designated disk drive based on the remotecontroller connection pointer 402. Then, the address of the diskcontroller (disk controller B 113) actually connected to the designateddisk drive and the address of the disk drive in the disk drive group Bconnected to the particular disk controller B 113 are acquired on thebasis of the remote controller-connected disk data 315 found as above.

Then, step 706 converts the address of the data to be read which hasbeen received in the read request into the format of the disk driveconnected to the disk controller B 113.

In a read/write request from the mainframe 101, the address of data tobe read or written is normally designated by the cylinder number, thehead number and the record number according to the CKD format.

The record address expressed by the cylinder number, the head number andthe record number will hereinafter be called CCHHR.

The disk drive connected to the disk controller B 113, on the otherhand, has an access interface designated by LBA (logical block address)in accordance with the FBA format.

Consequently, step 706 converts the access address of the data to beread from CKD format to FBA format.

The conversion formula is given, for example, by

LBA=(CC×number of heads+HH)×track length+record number×record length

According to this embodiment, the disk controller A 104 and the diskcontroller B 113 may have the same interface, in which case theconversion of the input/output interface format is not required.

Step 707 issues a request to the disk controller B 113 to read the datafrom the area of the corresponding disk drive calculated in step 706.

Step 708 waits for the arrival of the requested data from the diskcontroller B 113.

Step 709 sends the data received from the disk controller B 113 to themain frame 101 thereby to complete the process.

The disk controller B 113 simply reads the data requested by the diskcontroller A 104 from a disk drive, and sends it to the disk controllerA 104. This process, therefore, is not described specifically in theprocessing flow.

Next, an explanation will be given of a case in which data backed up inthe MT library system 116 is restored by the restore process 164 on themainframe 101 in the disk drive group B 114 of the open system of theprocessing system B through the disk controller A 104 and the mainframe101 of the processing system A.

As described already above, the mainframe 101 has recognized that thedisk drive group B 113 (disks C and D) are also connected to the diskcontroller A 104.

Therefore, no explanation will be given of the operation of themainframe 101 which is simply to issue a write request to the diskcontroller A 104 to write the data read from the MT library system 116.

Upon receipt of a write request from the mainframe 101, the diskcontroller A 104 executes the process in accordance with the flowchartof FIG. 8.

In the processing flow of FIG. 8, steps 800 to 801, 803 to 806 aresimilar to steps 700 to 701, 703 to 706 in FIG. 7, respectively, andtherefore will not be explained. Also, step 802 is normally the writeoperation, since the request from the mainframe 101 is a write request.Only the parts different from FIG. 7 will be described below.

Step 807 issues a request to the disk controller B 113 to write data inthe area of the corresponding disk drive calculated in step 807.

Next, in step 808, the write data is received from the mainframe 101 andsent to the disk controller B 113.

Then, step 809 waits for a report on the completion of the write requestfrom the disk controller B 113, and upon receipt of the completionreport, sends it to the mainframe 101 thereby to complete the process.

The disk controller B 113 simply reads the data requested by the diskcontroller A 104 from the corresponding disk drive and sends it to thedisk controller A 104. The related processing flow, therefore, is notshown specifically.

The foregoing description concerns a system for backing up data of thedisk drive group B 114 of the open system of the processing system B bythe processing system A. As another embodiment, a heterogeneous I/Osubsystem can be configured in which only the disk controller B and thedisk drive group B are connected to the processing system A and themainframe is connected with two I/O subsystems having differentinterfaces. In such a case, three or more instead of two I/O subsystemscan be connected.

The above-mentioned embodiment permits data to be backed up between I/Osubsystems having different access interfaces.

As a result, data stored in an I/O subsystem for an open system can bebacked up into an I/O subsystem for the mainframe.

Also, the back-up mechanism of the mainframe includes a large-capacity,high-performance and high-reliability MT library system. The data of theI/O subsystem for an open system, therefore, can be backed up by amainframe back-up mechanism high in performance and reliability.

Further, different I/O subsystems can be connected to the mainframe.

What is claimed is:
 1. A method of accessing data in an informationprocessing system having a host computer, a first disk subsystemconnected to said host computer, a separate second disk subsystemconnected to said first disk subsystem via a communication line, and ananother host computer connectable to said second disk subsystem themethod comprising the steps of: outputting an access request in avariable-length data format from said host computer to said first disksubsystem; receiving the access request in a variable-length data formatin a disk controller of said first disk subsystem and generating anaccess request in a fixed-length data format based on the receivedaccess request in a variable-length data format; outputting the accessrequest in a fixed-length data format from the disk controller of saidfirst disk subsystem to a disk controller of said second disk subsystemvia the communication line; accessing a disk of said second disksubsystem by said disk controller of said second disk subsystem inaccordance with the access request in a fixed-length data format; andtransferring data, which are indicated by the access request in afixed-length data format and which are stored by said another hostcomputer, from said disk controller of said second disk subsystem tosaid disk controller of said first disk subsystem via the communicationline.
 2. The method as claimed in claim 1, wherein the step ofgenerating an access request in a fixed-length data format comprises thesteps of: determining whether an access destination of the accessrequest is a disk in said second disk subsystem; and when the accessdestination of the access request is a disk in said second disksubsystem, generating an access request of a fixed-length data formatbased on the received access request in a variable-length data format.3. The method as claimed in claim 2, further comprising the step ofaccessing a disk of said first disk subsystem based on the accessrequest in a variable-length data format, when the access destination ofthe access request is a disk in said first disk subsystem.
 4. A diskcontroller comprising: a host I/O (input/Output) unit for sending andreceiving data in a variable-length data format to and from a hostcomputer; an inter-I/O subsystem data transfer unit for sending andreceiving data in a fixed-length data format to and from anotherseparate disk controller connectable to another host computer, via acommunication line; and a processing unit for converting, when an accessrequest in a variable-length data format received via said host I/O unithaving an access destination which is said another disk controller, anaccess request in a variable-length data format into an access requestin a fixed-length data format, and forwarding the access request in afixed-length data format to said inter-I/O subsystem data transfer unitto send to said another disk controller, via the communication line, andfor converting response data received at said inter-I/O subsystem datatransfer unit from said another disk controller, via the communicationline, in response to the access request in the fixed-length data formatinto data in a variable-length data format and for forwarding data inthe variable-length data format to said host I/O unit.
 5. The diskcontroller as claimed in claim 4, further comprising a storage unit forstoring information indicating whether a destination disk specified byan access request sent from said host computer is a disk connected tosaid disk controller and for storing other information indicating whichdisk of another disk controller is said destination disk connect ed toupon said destination disk not being connected to said disk controller.6. The disk controller as claimed in claim 4, wherein, when an accessdestination specified by an access request sent from said host computeris a disk connected to said disk controller, said processing unit sendsan access request to a disk connected to said disk controller based onan access request in a variable-length data format, and when an accessdestination specified by an access request sent from said host computeris a disk connected to said another disk controller, said processingunit generates an access request in a fixed-length data format based onan access request in a variable-length data format sent from said hostcomputer and sends said access request in said fixed-length data formatto a disk connected to said another disk controller.
 7. A heterogeneouscomputer system comprising: a host computer; a first I/O (Input/Output)subsystem including at least one external storage device; a second I/Osubsystem connected to said host computer and to said first I/Osubsystem, said second I/O subsystem including an external storagedevice to which said host computer makes an access request in avariable-length record format, and means for converting the accessrequest in the variable-length record format to an access request in afixed-length record format and forwarding the access request in thefixed-length format to said first I/O subsystem, when an accessdestination of said access request corresponds to the external storagedevice included in said first I/O subsystem; a variable-length dataformat interface arranged to connect said second I/O subsystem to saidhost computer; and a fixed-length data format interface arranged toconnect said first I/O subsystem to said second I/O subsystem, via acommunication line, and enable data transfer between said first I/Osubsystem and said second I/O subsystem, via the communication line. 8.The method as claimed in claim 3, further comprising connecting saidsecond disk subsystem to another host computer via a fixed-length dataformat interface.
 9. The method as claimed in claim 1, furthercomprising connecting said second disk subsystem to another hostcomputer via a fixed-length data format interface.
 10. The method asclaimed in claim 2, further comprising connecting said second disksubsystem to another host computer via a fixed-length data formatinterface.
 11. The disk controller as claimed in claim 4, wherein saidanother disk controller is connectable to another host computer via afixed-length data format interface.
 12. The disk controller as claimedin claim 5, wherein said another disk controller is connectable toanother host computer via a fixed-length data format interface.
 13. Thedisk controller as claimed in claim 6, wherein said another diskcontroller is connectable to another host computer via a fixed-lengthdata format interface.
 14. The heterogeneous computer system as claimedin claim 7, wherein said first I/O subsystem is connectable to anotherhost computer via a fixed-length data format interface.
 15. Aninformation processing system, comprising: a first host; a first disksubsystem coupled to the first host, and including at least a storagedevice to store data; a second host; a second disk subsystem coupled tothe second host, and connected to the first disk subsystem, via acommunication line, said second disk subsystem including at least astorage device to store data; wherein the first disk subsystem isconfigured, when an access request in a variable-length data format isissued from the first host, to generate an access request in afixed-length data format and to transfer the access request in thefixed-length data format to the second disk subsystem, via thecommunication line; and wherein the second disk subsystem is configured,when the access request in the fixed-length data format is received, viathe communication line, to access the storage device and to transferdata from the storage device as indicated by the access request in thefixed-length data format to the first disk subsystem, via thecommunication line.
 16. The information processing system as claimed inclaim 15, wherein the first disk subsystem comprises: an I/O unitarranged to send/receive data in the variable-length data format to/fromthe first host; a data transfer unit arranged to send/receive data inthe fixed-length data format to/from the second disk subsystem, via thecommunication line; and a processor unit arranged to convert the accessrequest in the variable-length data format received from the first host,via the I/O unit, into an access request in a fixed-length data formatand send to the data transfer unit, and to convert response datareceived from the data transfer unit in response to the access requestin the fixed-length data format into data in a variable-length dataformat and send to the I/O unit.
 17. The information processing systemas claimed in claim 16, wherein, when an access destination specified bythe access request in the variable-length data format sent from thefirst host indicates the storage device included in the first disksubsystem, the processor unit sends an access request to the storagedevice in the variable-length data format.
 18. The informationprocessing system as claimed in claim 17, wherein, when an accessdestination specified by the access request in the variable-length dataformat sent from the first host indicates the storage device included inthe second disk subsystem, the processor unit generates an accessrequest in a fixed-length data format and sends the access request inthe fixed-length data format to the storage device included in thesecond disk subsystem.
 19. The information processing system as claimedin claim 16, wherein the processor unit is configured to determinewhether an access destination specified by the access request in thevariable-length data format sent from the first host indicates thestorage device included in the second disk subsystem, and generate anaccess request in a fixed-length data format when the access destinationspecified by the access request indicates the storage device included inthe second disk subsystem.
 20. The information processing system asclaimed in claim 15, wherein the second disk subsystem is coupled to thesecond host, via a fixed-length data format interface.